MAS NMR to the Investigation of the Adsorption

J. Phys. Chem. B 1998, 102, 7023-7027

7023

Adaptation of 27Al CP/MAS NMR to the Investigation of the Adsorption of Molybdate Ions at the γ-Al2O3/Water Interface Dimitri Mertens de Wilmar and Olivier Clause Kinetics and Catalysis DiVision, Institut Franc¸ ais du Pe´ trole, 1-4, aVenue de Bois-Pre´ au, 92852 Rueil-Malmaison Cedex, France

Jean-Baptiste d’Espinose de la Caillerie* Laboratoire de Physique Quantique, URA CNRS 1428, Ecole Supe´ rieure de Physique et de Chimie Industrielles de la Ville de Paris, 10 rue Vauquelin, 75231 Paris Cedex 05, France ReceiVed: April 15, 1998; In Final Form: June 22, 1998

Recent studies have demonstrated the importance of the formation of a hydrated aluminum polymolybdate when a catalyst is prepared by impregnating γ-alumina with an aqueous solution of ammonium heptamolybdate [Carrier, X.; Lambert, J. F.; Che, M. J. Am. Chem. Soc. 1997, 119, 9, 10137-10146]. We show here how a simple {1H} 27Al CP MAS NMR experiment performed in the intermediate regime can easily reveal the formation of a hydrated aluminum polymolybdate on the surface of alumina. Taking, simultaneously, advantage of the differences in dipolar polarization transfer and those in quadrupolar effects on spin-locking efficiency, it is possible, after setting the experimental parameters on reference compounds, to partially suppress the signal resulting from the alumina support at 8.5 ppm while enhancing the 14 ppm resonance of the aluminum polymolybdate. In this way, the occurrence of a hydrated polymolybdate is evidenced, not only on samples impregnated with (NH4)6Mo7O24 but also with Na2MoO4. Furthermore, a dialysis experiment proved that a dissolution/reprecipitation mechanism is at least in part responsible for the formation of the Al-O-Mo bond.

Introduction Solid state nuclear magnetic resonance (NMR) is a proven method to study the local environments of active species dispersed on the surface of heterogeneous catalysts.1 One of its main advantage compared to most surface studies methods is that it does not require any preparation of the surface and is therefore totally noninvasive. On the downside it is a bulk sensitive method, and if the nuclei under study are present in the volume of the catalyst, the signal of the active species might be obscured. This has considerably limited the usefulness of solid state NMR for studying heterogeneous catalysts when the active or promoting species is not directly observable. A recent emphasis has appeared in the literature on the importance of dissolution/reprecipitation of mixed hydroxo species during the impregnation of alumina-supported catalysts. Such phenomena have been evidenced in systems of practical importance such as Ni(II), Co(II),2 and Mo(VI)3-5 on γ-alumina. Impregnations by paramagnetic Ni(II) or Co(II) ions are evidently not amenable to NMR studies, while for 95Mo NMR is difficult in the solid state due to its low gyromagnetic ratio and mediocre natural abundance (15.72%). This has so far limited the study of poorly crystallized Mo based catalysts to enriched samples.6 By contrast, 27Al has an excellent absolute sensitivity, 103 superior to 95Mo, and a 100% natural abundance. It is therefore tempting to study the Mo-alumina interaction from the aluminum standpoint, but this necessitates being able to discriminate the weak signal resulting from the few aluminum interacting with molybdenum. * Corresponding author: LPQ-ESPCI, 10, rue Vauquelin, 75321 Paris Cedex 05, France. Telephone: (33) 01 40 79 46 20. Fax: (33) 01 40 79 47 44. E-mail: [email protected].

Cross polarization (CP) from protons is a tool of choice for the observation of phenomena linked to the surface hydration of non-hydrous oxides. In this way, NMR is made surface selective and remains quantitative for spin 1/2 nuclei when spin dynamics is taken into account. The selective detection of surface hydroxylated species by proton CP to spin I ) 1/2 nuclei has been very widely used, for example, when the support is silica.7-9 However, for alumina supported catalysts, a straightforward analysis based on dipolar couplings is hampered by the quadrupolar nature of 27Al (I ) 5/2). We propose to take advantage of the dependence of the efficiency of the polarization transfer on quadrupolar interactions to simultaneously enhance the hydroxylated species and filter out the alumina signal. We show that we are able in this way to selectively reveal aluminum engaged in a hydroxylated mixed aluminum-molybdenum species during aqueous Mo(VI) impregnation of alumina. This finding is of some interest considering the importance of Mo/ Al2O3 materials as oxidation or olefin metathesis catalysts as well as precursors to hydrorefining catalysts.10 More precisely, the presence of aluminum ions in the deposited phase has an impact on the accessibility to reactants and resistance to thermal sintering of the supported phase, as was observed previously for the MgO/Al2O3 systems.11 Experimental Section Materials. Bayerite was courtesy of Dr. D. Coster. An ammonium hexamolybdenoaluminate reference compound was prepared by coaddition of a 0.1 M (NH4)6Mo7O24 and of a 0.1 M Al(NO3)3 solution. The pH was fixed at 3.8 by addition of diluted nitric acid. The resulting suspension was stirred for 3 h at 298 K and then filtered and dried at room temperature.

S1089-5647(98)01863-X CCC: $15.00 © 1998 American Chemical Society Published on Web 08/18/1998

7024 J. Phys. Chem. B, Vol. 102, No. 36, 1998 Chemical and thermogravimetric analyses and XRD are compatible with (NH4)3[Al(OH)6Mo6O18].12 Impregnation of powdered γ-alumina (Procatalyse, specific surface area 200 m2 g-1, mean pore diameter 6 nm; Na2O content lower than 40 ppm; ground and pretreated in oven for 3 h at 823 K) was conducted by stirring in an excess of solution at room temperature: Na2MoO4 (0.1 M), 0.5 h of contact, pH ) 3.9, 40 mL/g liquid/solid (sample AlMo-1, 7.0 Mo wt %); (NH4)6Mo7O24 (0.05 M), 0.5 h of contact, pH ) 3.8, 200 mL/g liquid/solid (sample AlMo-7, 25.0 Mo wt %). The suspensions were filtered and dried at room temperature. Alumina blanks were prepared following the same procedures but without the molybdenum precursors. Mo contents were determined by X-ray fluorescence. An impregnation was also performed using a dialysis membrane (1000 molecular weight cutoff). Two dialysis tubes containing 40 g of alumina spheres in distilled water were immersed in 2 L of a 0.05 M (NH4)6Mo7O24 aqueous solution. The pH was fixed at 3.8. The spheres were then filtered, dried, and ground (sample AlMo-9, 12.3 Mo wt %). The precipitate outside the tubes was also filtered and dried (sample AlMo9bis, 52.6 Mo wt % and 0.21 Al wt %). MAS NMR. Magic angle spinning nuclear magnetic resonance (MAS NMR) experiments were performed on a Bruker ASX500 spectrometer at 11.7 T in 4 mm zirconia rotors. 27Al one-pulse experiments were performed at 12 kHz with a selective pulse (π/6) duration of 0.5 µs, recycle time 1 s, and 160 acquisitions. The cross-polarization (CP) pulse sequence was the usual one with a recycle delay of 5 s, a contact time (tCP) of 500 µs, a 1H radio frequency magnetic field strength (ΩS/2π) of 56 kHz, and a spinning frequency (νr) of 8 kHz.13 2400 Acquisitions were performed, except for AlMo-9bis, where 102 000 scans were acquired. It was checked that the 27Al magnetization followed the phase of the 1H, thereby confirming that it resulted solely from cross polarization. The 27Al radio frequency (rf) field strength (ΩI/2π) was calibrated within 2 kHz by measuring the duration of a pulse in a 0.1 M aqueous solution of Al(NO3)3. The same solution was the chemical shift reference. Independently of CP, a two-pulse spin locking experiment was performed on 27Al: a selective π/(2I+1) pulse followed by a spin locking pulse of variable duration. Theoretical Background For quadrupolar nuclei such as 27Al, quadrupolar effects on spin locking efficiency and polarization transfer does not allow a simple discussion based on the evolution of the spectra under different Hartmann-Hahn matching and contact times. Recently, following the work of Vega,14,15 several papers attempting a theoretical description of the dynamics of cross polarization under MAS involving quadrupolar nuclei have been published and important facts have emerged. Level Matching. For CPMAS of half-integer quadrupolar spins coupled to spin 1/2 nuclei, the Hartmann-Hahn condition becomes

ΩS ) ωnut where ΩS is the strength of the radio frequency for the spin 1/2 nuclei (here 1H) and ωnut is the nutation angular frequency associated with the central transition (|1/2〉|-1/2〉) of the observed quadrupolar nuclei (here 27Al). For static experiments, if the quadrupolar frequency ωQ (ωQ ) 3e2qQ/2I(2I-1)p) is small, the quadrupolar interaction does not affect the nutation frequency and the matching condition is simply

Mertens de Wilmar et al.

ΩS ) ΩI

ΩI/ωQ . 1

if

with ΩI as the radio frequency strength for the quadrupolar spin. If the quadrupolar frequency is large compared to the radio frequency strength of the quadrupolar spin, a selective polarization transfer to the central transition is possible but the nutation angular frequency becomes

ωnut ) (I + 1/2)ΩI The matching condition is thus

ΩS ) (I + 1/2)ΩI

if

ΩI/ωQ , 1

Spin Locking. Besides energy level matching, cross polarization requires the locking of the (|1/2〉|-1/2〉) coherence. Vega has shown that the spin locking of quadrupolar half-integer spins under magic angle spinning (MAS) is efficient in the fast passage and adiabatic regime, that is for R , 1 and R . 1, respectively, where

R)

Ω2I ωQωr

with ωr being the rotation angular frequency of the rotor. Of fundamental interest to our studies, Sun et al. have described the efficiency of spin locking in the fast passage regime (R , 1).16 They showed that avoiding resonance conditions (ΩI * ωr), spin-locking efficiency is achieved for R < 0.02 if the second-order interaction can be neglected

ω2Q/ωO , ΩI (with ωO being the Larmor angular frequency). If second-order effects need to be taken into account, efficient spin locking requires even lower values of R. MAS and Nutation. Finally, Ding and McDowell underlined the influence of sample spinning on the nutation frequencies.17,18 This is further complicated by the effect of cross relaxation on nutation. Indeed the cross relaxation is governed by the heterodipolar interaction, which, in turn, is affected by the rotation of the sample. It follows that the efficiency of the polarization transfer depends in a complicated manner on the strength of the aluminum radio frequency field (ΩI) and on the spinning speed (ωr). This is exemplified on bayerite (Al(OH)3) (Figure 1) where two maxima appear corresponding to a selective (low ΩI) and nonselective excitation (high ΩI) as well as the possibility of excitation in the intermediate case. This at constant ωQ (there is only one Al site in bayerite) and ΩS. The dependence on the spinning speed of the cross polarized nutation frequencies due to the cross-relaxation effect on the nutation frequencies (neglecting the orientation dependence of the dipolar coupling) is also illustrated by varying the spinning speed. Therefore, depending on the experimental conditions, cross polarization efficiency can be expected to be vastly different for two sites of different dipolar and quadrupolar characteristics. It follows that in favorable cases one should be able to differentiate unresolved sites of close chemical shifts by varying the intensity of the Al rf field, playing thereby simultaneously on the Hartmann-Hahn (mis)matching and the efficiency of the Al spin locking. Due to homo- and heterodipolar coupling effects on cross relaxation and, more importantly, to the spin locking efficiency dependence on MAS, it is essential that the spinning speed remains constant for a meaningful interpretation

Adsorption of Molybdate Ions

J. Phys. Chem. B, Vol. 102, No. 36, 1998 7025

Figure 1. Variable ΩI proton CPMAS 27Al NMR experiment on bayerite. ΩS: 56 kHz. tcp: 500 µs. O: static b -: νr ) 5 kHz. sbs: νr ) 12 kHz.

Figure 3. One-pulse and proton CP (νr: 8 kHz. ΩS/2π: 56 kHz. tcp: 500 µs. ΩI/2π: 55 kHz.) 27Al MAS NMR of the coprecipitated reference ammonium hexamolybdenoaluminate.

Figure 2. One-pulse 27Al MAS NMR spectra of the alumina blank, of Mo impregnated alumina, and of the solid collected outside the dialysis membrane.

of the results. For the same reason, the contact time also must stay constant throughout the experiment. Results Model Compounds. Alumina exhibits an octahedral resonance at 8.5 ppm (Figure 2) and ammonium hexamolybdenoaluminate a single sharp line at 14 ppm (Figure 3). A preliminary experiment was performed on a mixture of hydrated alumina (9 ppm) and [Mo,Al] coprecipitate (14 ppm) at νr ) 8 kHz and varying the strength of the 27Al rf field (Figure 4). A similar experiment was also performed at 14 kHz, but the quality of the data was extremely poor, probably because at such speeds the cross polarization becomes very sensitive to small variations in Hartmann-Hahn mismatches due to instabilities of our amplifiers output.19 It appeared that the cross polarization of the two species differed vastly suggesting applicability of the method. The frequency of the maxima was only slightly different for the two lines indicating that the main effect was a difference in spin locking efficiency rather than in the nutation frequency. This was confirmed by a direct spinlock experiment (Figure 5) under the conditions of maximum polarization transfer for the non selective case as determined in Figure 4. It appeared that after 500 µs a third of the magnetization of the ammonium hexamolybdenoaluminate remained locked while the magnetization of the alumina had almost entirely dephased.

Figure 4. Variable ΩI proton CPMAS 27Al NMR experiment on a mixture of hydrated alumina (O: 8 ppm) and [Mo,Al] coprecipitate (b: 14 ppm). νr: 8 kHz. ΩS/2π: 56 kHz. tcp: 500 µs.

Figure 5. Variable spin locking duration 27Al MAS NMR experiment on a mixture of hydrated alumina (O: 8 ppm) and [Mo,Al] coprecipitate (b: 14 ppm). νr: 8 kHz. ΩI/2π: 55 kHz.

As a result, {1H f 27Al} CPMAS performed in the intermediate regime acts as a filter favoring the resonance of the ammonium hexamolybdenoaluminate or that of the alumina depending on the applied 27Al rf strength (ΩI). A complete theoretical treatment for CPMAS of quadrupolar nuclei not being

7026 J. Phys. Chem. B, Vol. 102, No. 36, 1998

Mertens de Wilmar et al. to fully characterize the formed species. Moreover, the sample collected outside the tube in the dialysis experiment (AlMo9bis) gave a spectra similar to the impregnated samples (Figure 6), thereby confirming the fact that a dissolution/reprecipitation mechanism is involved. Discussion

Figure 6. Proton CP MAS NMR 27Al spectra of the alumina blank, of Mo impregnated alumina, and of the solid collected outside the dialysis membrane. νr: 8 kHz. ΩS/2π: 56 kHz. tcp: 500 µs. ΩI/2π: 55 kHz.

available at the moment for the intermediate regime (R ≈ 1), the optimum ΩI and contact time tcp values have to be set empirically. To favor the aluminum polymolybdate resonance, they were set at ΩI/2π ) 55 kHz and tcp ) 500 µs on the basis of the results shown in Figures 4 and 5. Conversely, a ΩI/2π value of 70 kHz would suppress this resonance relative to the one of the hydrated alumina. The relative intensities of these two species therefore depend very sharply on relatively small variations of the 27Al radio frequency strength. The CP MAS experiments in this studies were thus performed under such conditions as to filter the aluminum polymolybdate, hoping to detect it even as a minor occurrence. Impregnated Aluminas. The one-pulse spectra of the impregnated samples are almost identical to the one of the blank experiment and typical of aluminas with an AlIV and an AlVI resonance at 70 and 8.5 ppm, respectively. A small contribution at 14 ppm is, however, discernible for sample AlMo-9 (Figure 2). It is therefore reasonable to assume that the octahedral resonance is actually the superposition of two unresolved contributions: the broad and intense AlVI alumina resonance and a weaker one resulting from a mixed [Mo,Al] hydroxide formed during impregnation. Nevertheless, even with the best efforts and the sharpest eyes, it was impossible on the basis of one-pulse 27Al MAS NMR alone to positively evidence the formation of a hydrated aluminum molybdate before the calcination on all samples. On the contrary, the {1H f 27Al} CP/MAS spectra obtained, as explained above, under conditions enhancing the 14 ppm line of the aluminum polymolybdate indicated in a totally unambiguous manner that this species formed in all cases (Figure 6). The recent work of Carrier et al has definitely shown that indeed, an Anderson-type heteropolymolybdate is formed in solution when alumina is impregnated by an aqueous solution of ammonium heptamolybdate, and that furthermore, this mixed [Al, Mo] species is also present on the surface of the uncalcined catalyst precursor.4-5 Actually, we obtained in that study similar spectra to the ones of Figure 6. Additionally, the data presented here tend to support the idea that the same type of aluminum molybdate also forms when the precursor is sodium molybdate, although it is not possible on the basis of the NMR data alone

Although the formation of Al2(MoO4) on calcined aluminasupported Mo catalysts has been reported and studied by 27Al MAS NMR for some time, the detection of the formation of a species involving the formation of a Al-O-Mo bond during the impregnation stage is fairly recent. Incidentally, Edwards and Decanio have already observed on rehydrated calcined catalysts a proton CP 27Al resonance at 13 ppm that they attributed to a compound formed by rehydration of aluminum molybdate on calcined Mo/Al2O3 catalysts.20,21 It must be emphasized that the usual straightforward comparative interpretation of one pulse and CP signals to distinguish bulk from surface species is obviously flawed when applied to quadrupolar nuclei.22,23 However, far from being a disadvantage, the complications arising from the quadrupolar interaction allows, at the price of careful and somewhat lengthy experimental settings, even more selective experiments. A rough back-of-the-envelope calculation easily demonstrates that more is involved here than simple surface signal enhancement. The structure of the γ-Al2O3 surface is not precisely known, but assuming a fully rehydrated surface, one would get a value of the order of 10 surface aluminum per nm2. With a BET surface area of 200 m2/g and within the conservative assumption that only surface atoms are cross polarized, the signal arises from about 3 mmol surface Al/g. On the other hand, all aluminum in [Al(OH)6Mo6O18]3- are hydroxylated and concerned by the polarization transfer. From a 10% Mo weight loading, if all Mo belonged to the aluminum heteropolymolybdate, one would get 0.2 mmol Al/g in the molybdate. The signal arising from the heteropolymolybdate would therefore remain 1 order of magnitude weaker than the one from the alumina surface. Assuming that in our materials νQ probably ranges from 0.1 to 1 MHz, based on typical values measured by SATRAS24 and off-resonance nutation,25 one can try to describe the relevant characteristic of our experiments. First, they were performed under nonselective conditions at ΩS ) ΩI. Second, they correspond to the transition between an adiabatic and intermediate regime of passage as R will range between 4 and 0.4 depending on ωQ. Third, resonance conditions were avoided as ΩI * ωr. Fourth, second order quadrupolar effects on spin locking need not be involved as ωQ2/ωO (ranging from about 100 Hz to 10 kHz in frequency units) was sufficiently smaller than ΩI. Consequently, on the basis of, for the most part, the second point, it was the differences in quadrupolar couplings affecting the spin locking, compounded with differences in heterodipolar couplings with protons (affecting simultaneously polarization transfer and nutation frequencies), which allowed discrimination of the two sites. Again, because of the intricate dependency of the signal on quadrupolar and dipolar couplings, we will not attempt a quantitative description of the CPMAS response but use the experiment as a qualitative filter. By performing the CP experiment in the intermediate regime, we are able not only to suppress the bulk alumina signal but also to reduce the relative contribution from the alumina surface. An important drawback is that the experimental conditions must be set empirically on model compounds. This implies that an a priori assumption must be made on the nature of the

Adsorption of Molybdate Ions species to be detected. However, the fact that we could evidence the previously undetected aluminum polymolybdate establishes the interest of such an 27Al CP experiment which we intend to perform on other metal-alumina catalysts systems. References and Notes (1) Bell, A. T., Pines, A., Eds. NMR Techniques in Catalysis; Marcel Dekker: New York, 1994. (2) Espinose de la Caillerie, J. B. d′; Kermarec, M.; Clause, O. J. Am. Chem. Soc. 1995, 117, 11471-11481. (3) Mertens de Wilmar, D. Thesis, Universite´ de Paris VI, 1998. (4) Carrier, X. Thesis, Universite´ de Paris VI, 1998. (5) Carrier, X.; Lambert, J. F.; Che, M. J. Am. Chem. Soc. 1997, 119, 10137-10146. (6) Edwards, J. C.; Adams, R. D.; Ellis, P. D. J. Am. Chem. Soc. 1990, 112, 8349-8364. Edwards, J. C.; Ellis, P. D. Langmuir 1991, 7, 21172134. (7) Maciel, G. E.; Ellis, P. D. In NMR Techniques in Catalysis; Bell, A. T., Pines, A., Eds.; Marcel Dekker: New York, 1994; Chapter 5, pp 231-309. (8) Hommel, H.; Legrand, A. P.; Dore´mieux, C.; Espinose de la Caillerie, J. B. d′ In The Surface Properties of Silica; Legrand, A. P., Ed.; Wiley: Chichester, 1998; Chapter 3B, pp 235-284.

J. Phys. Chem. B, Vol. 102, No. 36, 1998 7027 (9) Espinose de la Caillerie, J. B. d′; Kermarec, M.; Clause, O. J. Phys. Chem. 1995, 99, 17273-17281. (10) Gates, B. C. Catalytic Chemistry; Wiley: New York, 1992. (11) Rebours, B.; Espinose de la Caillerie, J.-B. d′; Clause, O. J. Am. Chem. Soc. 1994, 116, 1707-1717. (12) O ¨ hman, L. Inorg. Chem. 1989, 28, 3629-3632. (13) Pines, A.; Gibby, M. G.; Waugh, J. S. J. Chem. Phys. 1973, 59, 569-590. (14) Vega, A. J. J. Magn. Reson. 1992, 96, 50-68. (15) Vega, A. J. Solid State NMR 1992, 1, 17-32. (16) Sun, W.; Stephen, J. T.; Potter, L. D.; Wu, Y. J. Magn. Reson. 1995, A116, 181-188. (17) Ding, S.; McDowell, C. A. J. Magn. Reson. 1995, A112, 36-42. (18) Ding, S.; McDowell, C. A. J. Magn. Reson. 1995, A114, 80-87. (19) Marks, D.; Vega, S. J. Magn. Reson. 1996, A118, 157-172. (20) Edwards, J. C.; Decanio, E. C. Catal. Lett. 1993, 19, 121-130. (21) Han, O. H.; Lin, C. Y.; Haller, G. L. Catal. Lett. 1992, 14, 1-9. (22) Barrie, P. J. Chem. Phys. Lett. 1993, 208, 486-490. (23) Blumenfeld, A. J.; Coster, D. J.; Fripiat, J. J. Chem. Phys. Lett. 1994, 231, 491-498. (24) Kunath-Fandrei, G.; Bastow, T. J.; Hall, J. S.; Ja¨ger, C.; Smith, M. E. J. Phys. Chem. 1995, 99, 15138-15141. (25) Kraus, H.; Prins, R.; Kentgens, A. P. M. J. Phys. Chem. 1996, 100, 16336-16345.